Pressure heat treatment apparatus employed for preparing oxide superconducting wire

ABSTRACT

The method of preparing an oxide superconducting wire comprises steps of preparing a wire by coating raw material powder for a Bi—Pb—Sr—Ca—Cu—O based oxide superconductor including a 2223 phase with a metal and heat treating the wire in a pressurized atmosphere containing oxygen in a prescribed partial pressure, and the total pressure of the pressurized atmosphere is at least 0.5 MPa. The pressure heat treatment apparatus comprises a pressure furnace storing and heat treating a target in a pressurized atmosphere, a pressure regulator for measuring the total pressure in the pressure furnace, an oxygen concentration meter for measuring the oxygen concentration in the pressure furnace and a controller for controlling the oxygen partial pressure in the pressure furnace in response to the total pressure measured by the pressure regulator and the oxygen concentration measured by the oxygen concentration meter.

CROSS-REFERENCE TO RELATED PATENT APPLICATIONS

This application is a division of application Ser. No. 09/903,622, filedJul. 13, 2001, now pending, and based on Japanese Patent ApplicationsNo. 2000-213583, filed Jul. 14, 2000 and, No. 2001-038367, filed Feb.15, 2001 by Shinichi Kobayashi, Tetsuyuki Kaneko and Ryosuke Hata. Thisapplication claims only subject matter disclosed in the parentapplication and therefore presents no new matter.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a method of preparing an oxidesuperconducting wire and a pressure heat treatment apparatus employedfor the method, and more particularly, it relates to a method ofpreparing an oxide superconducting wire capable of preventing the wirefrom expansion in sintering and a pressure heat treatment apparatusemployed for the method.

2. Description of the Prior Art

Generally known is a method of preparing an oxide superconducting wireby heat treating a wire obtained by charging a metal tube with rawmaterial powder for an oxide superconductor and thereafter drawing orrolling the metal tube for sintering the raw material powder for theoxide superconductor. In the aforementioned heat treatment step forsintering, however, the wire is disadvantageously expanded to reducesuperconductivity of the obtained oxide superconducting wire.

Japanese Patent Laying-Open No. 5-101723 (1993) proposes a method ofpreparing an oxide superconducting wire by heat treating a metal tubecharged with powder of an oxide superconductor or a flat body thereofunder a pressurized atmosphere for sintering the powder of the oxidesuperconductor. This gazette describes that a wire having excellentsuperconductivity is obtained by pressure heat treatment.

More specifically, the metal tube charged with the powder of the oxidesuperconductor is stored in a heat-resistant and pressure-resistantclosed vessel to be prevented from expansion in sintering due to theinternal pressure increased following temperature increase in the closedvessel. The internal pressure can be obtained from a state equation ofgas or the like, and the aforementioned gazette describes that aninternal pressure of about 4 atm. can be obtained with a heatingtemperature of about 900° C., for example.

However, the internal pressure obtained following temperature increasein the closed vessel is only about 4 atm. (0.4 MPa), and it is difficultto sufficiently suppress expansion of the metal tube in sintering.

Further, the internal pressure varies with the temperature in the closedvessel, leading to pressure reduction in the process of temperatureincrease up to the sintering temperature and in the process oftemperature reduction to the room temperature after sintering.Therefore, expansion caused by gas generated at a temperature below thesintering temperature cannot be effectively prevented.

SUMMARY OF THE INVENTION

An object of the present invention is to provide a method of preparingan oxide superconducting wire capable of effectively preventingexpansion in a heat treatment step for sintering raw material powder foran oxide superconductor coated with a metal and obtaining an oxidesuperconducting wire having desired superconductivity and a pressureheat treatment apparatus employed for this method.

The method of preparing an oxide superconducting wire according to thepresent invention comprises steps of preparing a wire by coating with ametal raw material powder for a Bi—Pb—Sr—Ca—Cu—O based oxidesuperconductor containing bismuth, lead, strontium, calcium and copperand including a 2223 phase having composition ratios of (bismuth andlead), strontium, calcium and copper approximately expressed as 2:2:2:3and heat treating the wire in a pressurized atmosphere containing oxygenin a prescribed partial pressure, and the total pressure of thepressurized atmosphere is at least 0.5 MPa.

When the total pressure of the pressurized atmosphere is set to at least0.5 MPa as described above, the wire can be inhibited from expansioncaused by gas generated in the raw material powder for the oxidesuperconductor present in the wire.

In the aforementioned method of preparing an oxide superconducting wire,the total pressure of the pressurized atmosphere is preferably kept atleast 0.5 MPa from beginning to end of the heat treatment in the step ofheat treating the wire.

The wire can be inhibited also from expansion caused by gas generated inthe process of temperature increase and in the process of temperaturereduction by keeping the total pressure of the pressurized atmosphere atleast 0.5 MPa from the beginning of the process of temperature increaseto the end of the process of temperature reduction.

In the aforementioned method of preparing an oxide superconducting wire,the oxygen partial pressure in the pressurized atmosphere is preferablyat least 0.003 MPa and not more than 0.02 MPa. Further, the heattreatment temperature is preferably at least 800° C. and not more than840° C., and more preferably at least 810° C., and not more than 830° C.in the step of heat treating the wire.

Superconductivity of the oxide superconducting wire such as the criticalcurrent can be improved by defining the oxygen partial pressure and/orthe heat treatment temperature as described above.

The method of preparing an oxide superconducting wire according to thepresent invention preferably further comprises a step of preparing theraw material powder for the oxide superconductor by repeatingpulverization and heat treatment.

Further, the method of preparing an oxide superconducting wire accordingto the present invention preferably further comprises a step of heattreating the raw material powder for the oxide superconductor underdecompression and thereafter charging the raw material powder into ametal tube.

In the method of preparing an oxide superconducting wire according tothe present invention, the step of preparing the wire preferablyincludes an operation of drawing the metal tube thereby preparing a wirecoated with a metal. The step of preparing the wire preferably includesan operation of charging into another metal tube a plurality of wiresobtained by drawing the metal tube and thereafter performing drawing androlling on this metal tube thereby preparing a tape-like wire.

The method of preparing an oxide superconducting wire according to thepresent invention is preferably applied to preparation of a wireconsisting of a Bi—Pb—Sr—Ca—Cu—O based oxide superconductor,particularly optimum when employing raw material powder for an oxidesuperconductor having composition ratios of (Bi+Pb), Sr, Ca and Cuapproximately expressed as 2:2:2:3, and suitable for preparing a wire ofa Bi-based oxide superconductor including a 2223 phase having theaforementioned composition ratios, for example.

The pressure heat treatment apparatus employed for the method ofpreparing an oxide superconducting wire according to the presentinvention comprises a pressure heat treatment furnace storing a targetfor heat treating the target in a pressurized atmosphere, a pressuremeasuring device for measuring the total pressure in the pressure heattreatment furnace, an oxygen concentration measuring device formeasuring the oxygen concentration in the pressure heat treatmentfurnace and an oxygen partial pressure control part for controlling theoxygen partial pressure in the pressure heat treatment furnace inresponse to the total pressure measured by the pressure measuring deviceand the oxygen concentration measured by the oxygen concentrationmeasuring device.

In the pressure heat treatment apparatus having the aforementionedstructure, the oxygen partial pressure in the pressure heat treatmentfurnace can be precisely controlled. When this pressure heat treatmentapparatus is employed for the method of preparing an oxidesuperconducting wire according to the present invention, the oxygenpartial pressure in the pressurized atmosphere can be preciselycontrolled in the step of heat treating the wire, thereby readilyobtaining an oxide superconducting wire having superconductivity such asa desired critical current.

The pressure heat treatment apparatus according to the present inventionpreferably further comprises a gas introduction device for introducingoxygen gas or non-oxygen gas into the pressure heat treatment furnace inresponse to a control signal output from the oxygen partial pressurecontrol part.

In this case, the oxygen partial pressure in the pressure heat treatmentfurnace can be readily controlled by simply introducing oxygen gas ornon-oxygen gas into the pressure heat treatment furnace.

According to the inventive method of preparing an oxide superconductingwire, as hereinabove described, the wire can be effectively inhibitedfrom expansion in the heat treatment step, while superconductivity suchas the critical current can be improved by controlling the oxygenpartial pressure in the pressurized atmosphere within a prescribedrange. In the pressure heat treatment apparatus according to the presentinvention, further, the oxygen partial pressure in the pressure heattreatment furnace can be precisely controlled. When this pressure heattreatment apparatus is employed for the method of preparing an oxidesuperconducting wire according to the present invention, the oxygenpartial pressure in the pressurized atmosphere can be preciselycontrolled in the step of heat treating the wire, thereby readilyobtaining an oxide superconducting wire having superconductivity such asa desired critical current as a result.

The foregoing and other objects, features, aspects and advantages of thepresent invention will become more apparent from the following detaileddescription of the present invention when taken in conjunction with theaccompanying drawing.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 schematically illustrates the structure of a pressure heattreatment apparatus employed for a method of preparing an oxidesuperconducting wire according to the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

In the method disclosed in Japanese Patent Laying-Open No. 5-101723, themetal tube is conceivably prevented from expansion since the metal tubeis pressed from outside with the same pressure as that expanded in themetal tube when gas present in clearances (the filling factor isgenerally about 80% of the theoretical density before sintering) in thepowder of the oxide superconductor stored in the metal tube isvolume-expanded at a high temperature. If the metal tube is not expandedin this case, the volumes in the metal tube and the pressure vesselremain constant and hence the pressures in the metal tube and thepressure vessel storing the metal tube are changed when the sinteringtemperature is increased.

When the inventor studied the cause of expansion of such a wire,however, it was recognized that adsorbates such as carbon (C), water(H₂O) and oxygen (O₂) adhering to raw material powder for an oxidesuperconductor are vaporized during sintering to exert remarkableinfluence on volume expansion in a metal tube, leading to expansion ofthe wire. The inventor has discovered that the total pressure in apressurized atmosphere for heat treating the wire must be at least 5atm. (0.5 MPa) in order to suppress expansion caused not only by gaspresent in the clearances between grains forming the raw material powderfor the oxide superconductor but also by vaporization of adsorbatesadhering to the surfaces of the grains.

In a method of charging raw material powder for an oxide superconductorinto a metal tube and thereafter heat treating the obtained wire, thepowder adsorbs or contains various types of gas, and hence the gaspresent in the clearances between the grains forming the raw materialpowder is liberated during the heat treatment to expand the wire. Thisliberation of the gas and the expansion caused by the gas exert badinfluence on the oxide superconducting wire.

Also when the gas is homogeneously liberated as a whole in a smallquantity, the liberated gas penetrates into the clearances between theoxide superconducting crystal grains to hinder connection between thecrystal grains, i.e. electrical connection. Thus, the characteristic ofthe obtained oxide superconducting wire, particularly the criticalcurrent, is reduced.

In a tape-like oxide superconducting wire of 0.24 mm in thickness and3.2 to 3.6 mm in width having a number of oxide superconductor filamentsof 10 μm in thickness and 100 μm in width, for example, each oxidesuperconductor filament is formed by an aggregate of oxidesuperconductor crystal gains of 2 μm in thickness and 10 μm in width.While a current flows between the crystal grains of the oxidesuperconductor in this case, the path of the current is hindered whengas is present between the crystal grains.

When the gas is locally largely liberated, large expansion is caused todeform the shape of the wire itself on the part exposed to this gasliberation and locally reduce the critical current.

As means of suppressing the aforementioned liberation of gas, a methodof heat treating the wire with pressurization is proposed in JapanesePatent Laying-Open No. 5-101723, for example, as described above.However, this method, capable of suppressing expansion caused by gasliberation only when the gas is homogeneously liberated as a whole in asmall quantity, is insufficient for suppressing expansion caused by gasliberation when the gas is locally largely liberated.

In the method of preparing an oxide superconducting wire according tothe present invention, the wire can be effectively prevented fromexpansion caused by gas liberation whether the gas is homogeneouslyliberated as a whole in a small quantity or locally largely liberated bysetting the total pressure of the pressurized atmosphere to at least 0.5MPa.

In the method disclosed in Japanese Patent Laying-Open No. 5-101723, thepressure applied to the wire through the closed vessel varies with thetemperature in the closed vessel. When the wire is heat treated, gascomponents such as H₂O, CO₂ and O₂ are generated in the wire, i.e., inthe metal tube. H₂O, CO₂ and O₂ are generated in temperature regions of100 to 300° C., 700 to 800° C. and 800 to 900° C. respectively. In orderto suppress such gas components generated from the inner part of thewire by applying a pressure to the outer side of the wire, an externalpressure must be applied at a temperature of at least 100° C. In themethod disclosed in the aforementioned gazette, however, the pressure isreduced in response to the temperature in the closed vessel in theprocess of temperature increase up to the heat treatment temperature andin the process of temperature reduction after the heat treatment, andhence the wire cannot be prevented from expansion caused by generationof the aforementioned gas components in the temperature region below theheat treatment temperature.

In the method of preparing an oxide superconducting wire according tothe present invention, the total pressure of the pressurized atmosphereis preferably kept at least 0.5 MPa in the overall step of heat treatingthe wire, i.e., from the starting point of the heat treatment in theprocess of temperature increase to the end point of the heat treatmentin the process of temperature reduction. Thus, the total pressure of thepressurized atmosphere is kept at least 0.5 MPa in the starting point ofthe heat treatment within the temperature region of less than 100° C. aswell as in the end point of the heat treatment reaching the temperatureregion of less than 100° C., whereby the wire can be inhibited also fromexpansion caused by gas generated in a temperature region below the heattreatment temperature.

In the method of preparing an oxide superconducting wire according tothe present invention, the wire, heat treated in the pressurizedatmosphere of at least 5 atm. (0.5 MPa), can be inhibited from expansioncaused by gas generation in the wire. Particularly when the length ofthe wire exceeds 10 m, it is unconceivable that the gas is naturally andentirely liberated from end portions of the wire, and hence the effectaccording to the inventive method is further increased. Thus,superconductivity of the obtained oxide superconducting wire can also beimproved by suppressing expansion of the wire.

In relation to preparation of a bismuth-based oxide superconductorhaving a 2223 phase, it is known that the 2223 phase is stably generatedwhen the wire is heat treated under such a condition that the heattreatment atmosphere for sintering contains oxygen in the ratio of 20 to3% with respect to a total pressure of 1 atm. When pressure heattreatment is performed on the wire as disclosed in Japanese PatentLaying-Open No. 5-101723, however, the 2223 phase may not be stablygenerated even if the wire is heat treated in a pressurized atmospherewith a gas mixture containing oxygen in the aforementioned ratio. Whenthe powder of the oxide superconductor is sintered in a closed vesselhaving an internal pressure of about 4 atm. in a gas mixture containingabout 80% of nitrogen and about 20% of oxygen, i.e., in the air, theoxygen partial pressure reaches 0.8 atm. in response to the oxygencontent. The inventor has found that this oxygen pressure is out of acondition for stably generating the 2223 phase of the bismuth-basedoxide superconductor. The inventor has also found that the 2223 phase ofthe bismuth-based oxide superconductor can be stably generated when theoxygen partial pressure in the pressurized atmosphere is kept at 0.03 to0.2 atm. (0.003 to 0.02 MPa), even if the total pressure of thepressurized atmosphere is changed. In order to heat treat the wire in apressurized atmosphere of 10 atm., for example, a gas mixture containing0.3 to 2% of oxygen must be employed for performing heat treatment sothat the oxygen partial pressure is 0.03 to 0.2 atm. In order to stablygenerate the phase of the oxide superconductor, it is extremelyimportant to properly control the oxygen partial pressure condition alsowhen expansion can be suppressed by performing heat treatment in thepressurized atmosphere.

In the method of preparing an oxide superconducting wire according tothe present invention, therefore, the oxygen partial pressure in thepressurized atmosphere is preferably controlled to at least 0.003 MPaand not more than 0.02 MPa also when changing the total pressure of thepressurized atmosphere. In other words, the oxygen content in thepressurized atmosphere is preferably varied with the total pressure ofthe pressurized atmosphere, in order to control the oxygen partialpressure in the pressurized atmosphere within the aforementioned range.Thus, the 2223 phase of the oxide superconductor contained in the oxidesuperconducting wire can be stably generated.

As to the structure of the oxide superconducting wire, the periphery ofoxide superconductor filaments is coated with a metal such as silver ora silver alloy. Therefore, the oxygen partial pressure on the outer sideof the wire is hardly equivalent to the oxygen partial pressure aroundthe oxide superconductor filaments generated in the wire even ifcontrolled in the steps of preparing the oxide superconducting wire.However, the oxygen partial pressure in the wire can be forciblycontrolled to a desired value by maintaining the pressurized atmosphereunder conditions of an oxygen partial pressure desirable for generatingthe phase of the oxide superconductor and increasing the total pressureof the pressurized atmosphere. Therefore, the raw material powder forthe oxide superconductor is more densely sintered and the criticalcurrent of the oxide superconducting wire can be increased by increasingthe pressure of the atmosphere when heat treating the wire.

As hereinabove described, the oxygen partial pressure in the pressurizedatmosphere is more preferably controlled in the method of preparing anoxide superconducting wire according to the present invention. Thereaction for generating the target phase of the oxide superconductor canbe improved, the wire can be prevented from expansion in the pressureheat treatment, and superconductivity can be improved by adjusting theoxygen content in the pressurized atmosphere in response to the totalpressure in the pressurized atmosphere and controlling the oxygenpartial pressure to a desired level, i.e., at least 0.003 MPa and notmore than 0.02 MPa. Thus, the wire can be inhibited from localdeterioration caused by expansion and an oxide superconducting wireexcellent in superconductivity can be provided by employing theinventive method.

In the method of preparing an oxide superconducting wire according tothe present invention, the heat treatment temperature is more preferablycontrolled in response to the oxygen partial pressure in the pressurizedatmosphere. The heat treatment temperature is preferably controlledwithin the range of at least 780° C. and not more than 850° C. withrespect to the oxygen partial pressure of at least 0.003 MPa and notmore than 0.02 MPa, while the heat treatment temperature is morepreferably controlled within the range of at least 800° C. and not morethan 840° C. and further preferably controlled within the range of atleast 810° C. and not more than 830° C. in order to improvesuperconductivity such as the critical current.

FIG. 1 schematically shows the structure of a pressure heat treatmentapparatus employed for the method of preparing an oxide superconductingwire according to the present invention. As shown in FIG. 1, a pressurefurnace 1 serving as a pressure heat treatment furnace stores a targetsuch as a wire for pressurizing/heat treating the target. A closedvessel such as a high-temperature isostatic pressing (HIP) vessel or thelike may be employed as the pressure furnace 1. An exhaust valve 2discharges gas from the pressure furnace 1. A pressure regulator 3regulates and measures the total pressure in the pressure furnace 1. Agas delivery valve 4 delivers the gas from the pressure furnace 1, sothat an oxygen concentration meter 5 measures the oxygen concentrationthereof. An electric signal corresponding to the value of the totalpressure in the pressure furnace 1 regulated and measured by thepressure regulator 3 and an electric signal corresponding to the oxygenconcentration in the gas stored in the pressure furnace 1 measured bythe oxygen concentration meter 5 are transmitted to a controller 12. Thecontroller 12 outputs a control signal for controlling the oxygenpartial pressure in the pressure furnace by PID (proportional integraland differential) control in response to the aforementioned two electricsignals, and supplies the control signal to an oxygen flow meter 7 and anitrogen flow meter 10. If the oxygen partial pressure calculated fromthe measured total pressure and oxygen concentration is less than aprescribed set value, a necessary amount of oxygen gas regulated by theoxygen flow meter 7 is fed into the pressure furnace 1 from an oxygencylinder 8 through an oxygen introduction valve 7 until the oxygenpartial pressure reaches the prescribed set value. If the aforementionedcalculated oxygen partial pressure is in excess of the prescribed setvalue to the contrary, a necessary amount of nitrogen gas regulated bythe nitrogen flow meter 10 in response to the control signal is fed intothe pressure furnace 1 from a nitrogen cylinder 11 through a nitrogenintroduction valve 9. The oxygen gas or nitrogen gas must be fed whilecontrolling the total pressure in the pressure furnace 1 and maintainingthe same at a constant value by the pressure regulator 3.

The pressure heat treatment apparatus having the aforementionedstructure is employed for pressurizing/heat treating a wire prepared bycoating raw material powder for a Bi—Pb—Sr—Ca—Cu—O based oxidesuperconductor including a 2223 phase with a metal. This wire is storedin the pressure furnace 1. The pressure regulator 3 keeps the totalpressure in the pressure furnace 1 at least 0.5 MPa from a startingpoint of heat treatment in the process of temperature increase in thepressure furnace 1 to an end point of heat treatment in the process oftemperature reduction in the pressure furnace 1. A necessary amount ofoxygen gas regulated by the oxygen flow meter 7 is fed from the oxygencylinder 8 into the pressure furnace 1 through the oxygen introductionvalve 6 or a necessary amount of nitrogen gas regulated by the nitrogenflow meter 10 is fed into the pressure furnace 1 from the nitrogencylinder 11 through the nitrogen introduction valve 9 until the oxygenpartial pressure calculated from the total pressure regulated by thepressure regulator 3 and the oxygen concentration measured by the oxygenconcentration meter 5 reaches a prescribed value within the range of atleast 0.003 MPa and not more than 0.02 MPa. When the oxygen partialpressure reaches the prescribed value, the oxygen introduction valve 6and the nitrogen introduction valve 9 are closed. Thereafter the wire isheat treated.

EXAMPLE 1

Powder materials of compounds Bi₂O₃, PbO, SrCO₃, CaCO₃ and CuO weremixed with each other to prepare a powder mixture containing Bi, Pb, Sr,Ca and Cu in the ratios 1.82:0.33:1.92:2.01:3.02 as raw material powderfor a bismuth-based oxide superconductor. This powder mixture was heattreated at a temperature of 750° C. for 10 hours, and thereafter furtherheat treated at a temperature of 800° C. for 8 hours. The heat treatedpowder mixture was pulverized in an automatic mortar. The powderobtained by this pulverization was heat treated at a temperature of 850°C. for 4 hours, and thereafter pulverized in the automatic mortar again.

The powder obtained in the aforementioned manner was heat treated underdecompression, and thereafter charged into a silver pipe of 36 mm inouter diameter and 30 mm in inner diameter. Then, the silver pipecharged with the powder was drawn. 61 wires obtained in this manner werebundled and engaged in a silver pipe of 36 mm in outer diameter and 31mm in inner diameter, which in turn was subjected to drawing androlling. The rolling was performed with a draft of about 80%. Thus,tape-like wires of 0.25 mm in thickness and 3.6 mm in width wereobtained.

The wires of the bismuth-based oxide superconductor having a 2223 phaseprepared in the aforementioned manner were superposed with each otherthrough a clearance member of ceramic paper consisting of a mixture ofalumina fiber and zirconia powder, and wound on a spool of a stainlessalloy having a diameter of 50 cm. A prescribed pressure was applied tothe superposed wires and thereafter heating was started to heat treatthe superposed wires under prescribed conditions. After the heattreatment, the wires were cooled to the room temperature, and thereafterthe pressure was removed.

As shown in Table 1, the total pressure of the pressurized atmospherewas varied in the range of 0.1 to 20.0 MPa, for performing first heattreatment on the wires of respective samples. In each sample, the oxygenpartial pressure, the heat treatment temperature and the heat treatmenttime were set to 0.02 MPa, 840° C. and 50 hours respectively. Thereafterthe wire of each sample was rolled with a rolling reduction of about15%, and thereafter the total pressure of the pressurized atmosphere wasvaried in the range of 0.1 to 20.0 MPa as shown in Table 1 forperforming second heat treatment. In each sample, the oxygen partialpressure, the heat treatment temperature and the heat treatment timewere set to 0.02 MPa, 835° C. and 50 hours respectively. After the heattreatment, the degree of expansion of the wires was evaluated and thecritical current was measured at a temperature of 77 K. Table 1 alsoshows the results.

TABLE 1 Oxygen Heat Total Partial Heat Treatment Treatment Critical Ex-Sam- Pressure Pressure Temperature ( ) Time Current pansion ple (MPa)(MPa) first second (Hr) (A) of Wire 1 0.1 0.02 840 835 50 45 yes 2 0.20.02 840 835 50 45 yes 3 0.3 0.02 840 835 50 46 yes 4 0.4 0.02 840 83550 47 yes 5 0.5 0.02 840 835 50 47 no 6 0.8 0.02 840 835 50 49 no 7 1.00.02 840 835 50 55 no 8 1.5 0.02 840 835 50 56 no 9 2.0 0.02 840 835 5065 no 10 3.0 0.02 840 835 50 68 no 11 5.0 0.02 840 835 50 70 no 12 10.00.02 840 835 50 82 no 13 20.0 0.02 840 835 50 98 no

It is clearly understood from Table 1 that the degree of expansion ofthe wire depends on the total pressure of the pressurized atmospheresuch that no effect of suppressing expansion of the wire is attained ifthe total pressure of the pressurized atmosphere is not more than 0.4MPa while an effect of suppressing expansion of the wire is attained ifthe total pressure exceeds 0.5 MPa. It is also understood that thecritical current of the wire is also improved as the total pressure ofthe pressurized atmosphere is increased.

In the aforementioned Example 1, the heat treatment is performed twicein order to attain desired superconductivity, i.e., a desired criticalcurrent. In relation to confirmation of the effect of suppressingexpansion of the wire, it has been confirmed that no effect ofsuppressing expansion of the wire is attained if the total pressure ofthe pressurized atmosphere is not more than 0.4 MPa while the effect ofsuppressing expansion of the wire is attained if the total pressureexceeds 0.5 MPa also when the heat treatment is performed once.

EXAMPLE 2

Tape-like wires of a bismuth-based oxide superconductor were preparedsimilarly to Example 1. The wires were superposed with each otherthrough a clearance member of ceramic paper consisting of a mixture ofalumina fiber and zirconia powder, and the superposed wires were woundon a spool of a stainless alloy having a diameter of 50 cm. Thereafterpressurization starting conditions and depressurization startingconditions were varied as shown in Table 2, for heat treating the wiresof respective samples with a total pressure of 1.0 MPa, an oxygenpartial pressure of 0.02 MPa and a heat treatment temperature of 840° C.for a heat treatment time of 50 hours. After the heat treatment, thedegrees of expansion of the wires were evaluated. Table 2 also shows theresults.

TABLE 2 Oxygen Heat Heat Total Partial Treatment TreatmentPressurization Depressurization Ex- Pressure Pressure Temperature TimeStarting Condition Starting Condition pansion Sample (MPa) (MPa) ( )(Hr) (temperature) (temperature) of Wire 14 1.0 0.02 840 50 beforetemperature after cooled to room no increase (29) temperature (30) 151.0 0.02 840 50 during temperature after cooled to room yes increase(500) temperature (30) 16 1.0 0.02 840 50 during heat after cooled toroom yes treatment (840) temperature (30) 17 1.0 0.02 840 50 duringtemperature after cooled to room yes reduction (800) temperature (30) 181.0 0.02 840 50 before temperature during heat yes increase (29)treatment (840) 19 1.0 0.02 840 50 during temperature during heat yesincrease (500) treatment (840) 20 1.0 0.02 840 50 during heat duringheat yes treatment (840) treatment (840) 21 1.0 0.02 840 50 duringtemperature during heat yes reduction (800) treatment (840) 22 1.0 0.02840 50 before temperature in the process of yes increase (29)temperature reduction (700) 23 1.0 0.02 840 50 during temperature in theprocess of yes increase (500) temperature reduction (700) 24 1.0 0.02840 50 during heat in the process of yes treatment (840) temperaturereduction (700) 25 1.0 0.02 840 50 during temperature in the process ofyes reduction (800) temperature reduction (700)

It is clearly understood from Table 2 that an effect of suppressingexpansion of the wire can be attained in the processes of temperatureincrease and temperature reduction by starting pressurization beforeincreasing the temperature and starting depressurization after coolingthe wire to the room temperature in the step of heat treating the wirein a pressurized atmosphere. In other words, it is understood that thewire is expanded if the total pressure of the pressurized atmosphere isnot kept in the prescribed range of at least 0.5 MPa from the process oftemperature increase as the start point of the heat treatment in thepressurized atmosphere and to the process of temperature reduction asthe end point of the heat treatment.

EXAMPLE 3

Tape-like wires of a bismuth-based oxide superconductor were preparedsimilarly to Example 1. The wires were superposed with each otherthrough a clearance member of ceramic paper consisting of a mixture ofalumina fiber and zirconia powder, and the superposed wires were woundon a spool of a stainless alloy having a diameter of 50 cm. A prescribedpressure was applied to the superposed wires and thereafter heating wasstarted to heat treat the superposed wires under prescribed conditions.After the heat treatment, the wires were cooled to the room temperature,and thereafter the pressure was removed.

First heat treatment was performed on the wires of respective samples ina pressurized atmosphere having a total pressure of 1.0 MPa and anoxygen partial pressure of 0.005 MPa at a temperature of 810° C. for 50hours. Thereafter rolling was performed on the wires with a rollingreduction of about 15%, and then second heat treatment was performedwhile varying the oxygen partial pressure in the pressurized atmospherein the range of 0.2 to 0.001 MPa, as shown in Table 3. After the heattreatment, the degree of expansion of the wires was evaluated and thecritical current was measured at a temperature of 77 K. Table 3 alsoshows the results.

TABLE 3 Oxygen Heat Heat Total Partial Treatment Treatment Critical Ex-Sample Pressure Pressure Temperature Time Current pansion No. (MPa)(MPa) (° C.) (Hr) (A) of Wire 26 1.0 0.2 835 50  0 no 27 1.0 0.02 835 5045 no 28 1.0 0.018 832 50 60 no 29 1.0 0.01 815 50 65 no 30 1.0 0.005805 50 71 no 31 1.0 0.003 800 50 68 no 32 1.0 0.001 795 50 51 no

It is clearly understood from Table 3 that the wire was inhibited fromexpansion in each sample. This is because the total pressure of thepressurized atmosphere was 1.0 MPa.

While it was impossible to confirm superconduction in the sample No. 26heat treated with the oxygen partial pressure of 0.2 MPa, the samplesNos. 27 to 32 heat treated in the pressurized atmospheres having oxygenpartial pressures of not more than 0.02 MPa exhibited excellentsuperconductivity with high critical currents. It is understood that ahigher critical current can be obtained with a lower heat treatmenttemperature as the oxygen partial pressure is reduced, regardless of thetotal pressure of the pressurized atmosphere. It is also understood thatthe heat treatment temperature is preferably in the range of 800 to 830°C., in order to obtain a relatively high critical current. The sampleNo. 32 heat treated in the pressurized atmosphere having an oxygenpartial pressure of less than 0.003 MPa (0.001 MPa) exhibited a lowercritical current as compared with the samples Nos. 28 to 31 heat treatedin the pressurized atmospheres having oxygen partial pressures of atleast 0.003 MPa.

EXAMPLE 4

Tape-like wires of a bismuth-based oxide superconductor were preparedsimilarly to Example 1. The wires were superposed with each otherthrough a clearance member of ceramic paper consisting of a mixture ofalumina fiber and zirconia powder, and the superposed wires were woundon a spool of a stainless alloy having a diameter of 50 cm. A prescribedpressure was applied to the superposed wires and thereafter heating wasstarted to heat treat the superposed wires under prescribed conditions.After the heat treatment, the wires were cooled to the room temperature,and thereafter the pressure was removed.

First heat treatment was performed on the wires of respective samples ina pressurized atmosphere having a total pressure of 2.0 MPa and anoxygen partial pressure of 0.005 MPa at a temperature of 810° C. for 50hours. Thereafter rolling was performed on the wires with a rollingreduction of about 15%, and then second heat treatment was performedwhile varying the oxygen partial pressure in the pressurized atmospherein the range of 0.2 to 0.001 MPa, as shown in Table 4. After the heattreatment, the degree of expansion of the wires was evaluated and thecritical current was measured at a temperature of 77 K. Table 4 alsoshows the results.

TABLE 4 Oxygen Heat Heat Total Partial Treatment Treatment Critical Ex-Sample Pressure Pressure Temperature Time Current pansion No. (MPa)(MPa) (° C.) (Hr) (A) of Wire 33 2.0 0.2 835 50  0 no 34 2.0 0.02 830 5063 no 35 2.0 0.01 815 50 68 no 36 2.0 0.008 810 50 70 no 37 2.0 0.001795 50 43 no

It is clearly understood from Table 4 that the wire was inhibited fromexpansion in each sample. This is because the total pressure of thepressurized atmosphere was 2.0 MPa.

While it was impossible to confirm superconduction in the sample No. 33heat treated with the oxygen partial pressure of 0.2 MPa, the samplesNos. 34 to 37 heat treated in the pressurized atmospheres having oxygenpartial pressures of not more than 0.02 MPa exhibited excellentsuperconductivity with high critical currents. It is understood that ahigher critical current can be obtained with a lower heat treatmenttemperature as the oxygen partial pressure is reduced, regardless of thetotal pressure of the pressurized atmosphere. It is also understood thatthe heat treatment temperature is preferably in the range of 810 to 830°C., in order to obtain a relatively high critical current. The sampleNo. 37 heat treated in the pressurized atmosphere having an oxygenpartial pressure of less than 0.003 MPa (0.001 MPa) exhibited a lowercritical current as compared with the samples Nos. 34 to 36 heat treatedin the pressurized atmospheres having oxygen partial pressures of atleast 0.003 MPa.

EXAMPLE 5

Tape-like wires of a bismuth-based oxide superconductor were preparedsimilarly to Example 1. The wires were superposed with each otherthrough a clearance member of ceramic paper consisting of a mixture ofalumina fiber and zirconia powder, and the superposed wires were woundon a spool of a stainless alloy having a diameter of 50 cm. A prescribedpressure was applied to the superposed wires and thereafter heating wasstarted to heat treat the superposed wires under prescribed conditions.After the heat treatment, the wires were cooled to the room temperature,and thereafter the pressure was removed.

First heat treatment was performed on the wires of respective samples ina pressurized atmosphere having a total pressure of 3.0 MPa and anoxygen partial pressure of 0.005 MPa at a temperature of 810° C. for 50hours. Thereafter rolling was performed on the wires with a rollingreduction of about 15%, and then second heat treatment was performedwhile varying the oxygen partial pressure in the pressurized atmospherein the range of 0.2 to 0.001 MPa, as shown in Table 5. After the heattreatment, the degree of expansion of the wires was evaluated and thecritical current was measured at a temperature of 77 K. Table 5 alsoshows the results.

TABLE 5 Oxygen Heat Heat Total Partial Treatment Treatment Critical Ex-Sample Pressure Pressure Temperature Time Current pansion No. (MPa)(MPa) (° C.) (Hr) (A) of Wire 38 3.0 0.2 835 50  0 no 39 3.0 0.02 830 5065 no 40 3.0 0.015 812 50 70 no 41 3.0 0.012 810 50 72 no 42 3.0 0.005800 50 68 no 43 3.0 0.001 795 50 45 no

It is clearly understood from Table 5 that the wire was inhibited fromexpansion in each sample. This is because the total pressure of thepressuized atmosphere was 3.0 MPa.

While it was impossible to confirm superconduction in the sample No. 38heat treated with the oxygen partial pressure of 0.2 MPa, the samplesNos. 39 to 43 heat treated in the pressurized atmospheres having oxygenpartial pressures of not more than 0.02 MPa exhibited excellentsuperconductivity with high critical currents. It is understood that ahigher critical current can be obtained with a lower heat treatmenttemperature as the oxygen partial pressure is reduced, regardless of thetotal pressure of the pressurized atmosphere. It is also understood thatthe heat treatment temperature is preferably in the range of 800 to 830°C., in order to obtain a relatively high critical current. The sampleNo. 43 heat treated in the pressurized atmosphere having an oxygenpartial pressure of less than 0.003 MPa (0.001 MPa) exhibited a lowercritical current as compared with the samples Nos. 39 to 42 heat treatedin the pressurized atmospheres having oxygen partial pressures of atleast 0.003 MPa.

EXAMPLE 6

Tape-like wires of a bismuth-based oxide superconductor were preparedsimilarly to Example 1. The wires were superposed with each otherthrough a clearance member of ceramic paper consisting of a mixture ofalumina fiber and zirconia powder, and the superposed wires were woundon a spool of a stainless alloy having a diameter of 50 cm. A prescribedpressure was applied to the superposed wires and thereafter heating wasstarted to heat treat the superposed wires under prescribed conditions.After the heat treatment, the wires were cooled to the room temperature,and thereafter the pressure was removed.

First heat treatment was performed on the wires of respective samples ina pressurized atmosphere having a total pressure of 1.0 MPa and anoxygen partial pressure of 0.01 MPa at a temperature of 810° C. for 50hours. Thereafter rolling was performed on the wires with a rollingreduction of about 15%, and then second heat treatment was performedwhile varying the heat treatment temperature in the range of 845 to 805°C., as shown in Table 6. After the heat treatment, the degree ofexpansion of the wires was evaluated and the critical current wasmeasured at a temperature of 77 K. Table 6 also shows the results.

TABLE 6 Oxygen Heat Heat Total Partial Treatment Treatment Critical Ex-Sample Pressure Pressure Temperature Time Current pansion No. (MPa)(MPa) ( ) (Hr) (A) of Wire 44 1.0 0.01 830 50 24 no 45 1.0 0.01 825 5047 no 46 1.0 0.01 820 50 60 no 47 1.0 0.01 815 50 65 no 48 1.0 0.01 81050 52 no 49 1.0 0.01 805 50 40 no 50 1.0 0.02 845 50 38 no 51 1.0 0.02840 50 50 no 52 1.0 0.02 835 50 58 no 53 1.0 0.02 830 50 56 no 54 1.00.02 825 50 48 no 55 1.0 0.02 820 50 41 no

It is clearly understood from Table 6 that the wire was inhibited fromexpansion in each sample. This is because the total pressure of thepressurized atmosphere was 1.0 MPa.

When the oxygen partial pressure was 0.01 MPa, the samples Nos. 46 to 48heat treated at the temperatures of 810 to 820° C. exhibited relativelyhigh critical currents. When the oxygen partial pressure was 0.02 MPa,the samples Nos. 51 to 53 heat treated at the temperatures of 830 to840° C. exhibited relatively high critical currents.

Although the present invention has been described and illustrated indetail, it is clearly understood that the same is by way of illustrationand example only and is not to be taken by way of limitation, the spiritand scope of the present invention being limited only by the terms ofthe appended claims.

What is claimed is:
 1. A pressure heat treatment apparatus comprising: apressure heat treatment furnace storing a target for heat treating saidtarget in a pressurized atmosphere; a pressure regulator for measuringthe total pressure in said pressure heat treatment furnace and forregulating same; oxygen gas feeding means for feeding oxygen gas intosaid pressure heat treatment furnace; oxygen concentration measuringmeans for measuring the oxygen concentration in said pressure heattreatment furnace; and oxygen partial pressure control means, connectedto receive an input from said pressure regulator and an input from saidoxygen concentration measuring means and providing a control signal tosaid oxygen gas feeding means, for controlling the oxygen partialpressure in said pressure heat treatment furnace within a prescribedrange in response to the total pressure measure by said pressureregulator and the oxygen concentration measured by said oxygenconcentration measuring means, wherein said oxygen partial pressurecontrol means operatively cooperating with said pressure regulator sothat said total pressure is maintained equal to or greater than 0.5 MPafrom the beginning to the end of heat treating said target whilemaintaining said oxygen concentration within said prescribed range. 2.The pressure heat treatment apparatus as recited in claim 1 wherein saidoxygen partial pressure control means comprises a PID controller.
 3. Thepressure heat treatment apparatus as recited in claim 1 furthercomprising non-oxygen gas feeding means for feeding non-oxygen gas tosaid heat treatment furnace.
 4. The pressure heat treatment apparatus asrecited in claim 3 wherein said oxygen partial pressure control meanscontrols both said oxygen gas feeding means and said non-oxygen gasfeeding means.
 5. The pressure heat treatment apparatus as recited inclaim 4 wherein said oxygen partial pressure control means comprises aPID controller.